124 Bulletin 233. 



The full-grown larvre eat through the epidermal floor or roof of their 

 mines and drop to the ground where, about an inch below the surface, 

 they make small, thin, elongate, cylindrical, brown, papery cocoons in 

 which they transform early in May, through tender, whitish pupae into 

 the black adults or sawflies. 



Food plants. — In Europe this insect is recorded as feeding on Eng- 

 lish and Scotch elms. In this country. Dr. Felt reports it on these trees 

 and on American elms also. On the Cornell Campus there is a case where 

 American and Scotch elms grew so near that their branches often mingle, 

 and although the sawflies are often seen on the leaves of both trees, yet 

 not a leaf on the native trees are " blistered " by the insect while the 

 foreign trees are badly infested. In this locality the American elm seems 

 to be almost entirely exempt from the pest. The Camperdown variety of 

 Scotch elms is often infested. 



Work and destructiveness of th2 insect. — Its work is c^uite con- 

 spicuous, as is shown in Fig. 26. Twenty or more of the larvae often 

 mine in a single elm leaf, and their mines soon coalesce forming a large 

 " blister " often involving the whole leaf. Many mines just begun are 

 shown in the leaf in lower left-hand corner of Fig. 26, and larvae can be 

 seen at work in larger mines in the leaf in the right-hand corner of this 

 figure. Oftentimes a mine begun near the midrib of the leaf is confined 

 to the area between two large veins until it gets nearer the outer edge 

 where it extends under the smaller veins or into neighboring mines. Th.e 

 whole interior of the leaf is eaten, leaving only the outer epidermis which 

 soon turns brown. The " blisters "" arc nearly as conspicuous from 

 the lower as from the upper side of the leaf. The mines of its near 

 relative in alder (Fig. 29), scarcely show from the underside of the leaf, 

 possibly because the alder leaf seems thicker than the elm. 



In July, after the larvre leave the " blisters " on the elm leaves, the 

 mined areas bleach out to a dirty whitish color, sh.rivcl and curl. The 

 picture of an infested branch in Fig. 26 was taken at this stage. Unless 

 the leaves are wholly mined out, most of them remain on the tree a 

 considerable time longer, the mines often becoming holes. Infested trees 

 present the worst appearance about July ist, or soon after the larvae 

 disappear. As many of the leaves (!rop ofif as the new growth comes 

 on, and as no otlier broods of the insect appear, the infested trees begin 

 to recover by the end of July, and by September 1st, trees which were 

 badly infested in July often show but little signs of the insect's work to 

 the casual observer. 



I have seen small trees almost defoliated, an 1 thus stunted and 

 rendered unsightly by this sawfly miner, and from one-half to two-thirds 

 of the leaves on several large trees on the Cornell Campus have been 

 badly infested for several years. These large trees present a very ragged 



